The Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), are standardized by the 3GPP working groups. An agreed objective of the carrier aggregation (CA) work item in Release-11 of the LTE standard is the support of multiple timing advance commands for uplink (UL) CA in scenarios where UL timing synchronization for a secondary serving cell (SCell) is sufficiently different from that of the primary serving cell (PCell). Examples of such scenarios include simultaneous (parallel) UL transmission to a macro Evolved Node B (eNB) on one carrier frequency and to a lower power node such as a remote radio head (RRH) on a different intra/inter-band carrier frequency. Consequently, it was agreed to partition aggregated serving cells into one or more timing advance groups (TAGs) according to their UL synchronization requirements. For each TAG, only one serving cell is configured to perform the random access (RA) procedure in order to obtain a timing advance (TA) command, which in turn, is applicable to all serving cells in the TAG. The PCell performs the RA procedure for its TAG. A SCell is configured for the RA procedure in a TAG consisting only of SCells, which is hereafter denoted as a “SCell-TAG.”
Several physical layer aspects need to be considered when a user equipment (UE) receives multiple TA commands for different TAGs. These aspects include:                the permitted combinations of a Physical Random Access Channel (PRACH) transmission on a SCell and transmission of other UL channels and signals (such as Physical Uplink Control Channel (PUCCH), Physical Uplink Shared Channel (PUSCH), and Sounding Reference Symbol (SRS)) on a different serving cell;        the priorities within a permitted combination pertaining to a PRACH transmission on a SCell and transmission of other UL channels and signals (such as PUCCH, PUSCH, and SRS) on a different serving cell; and        power limitation and power fluctuation during transmission in one subframe due to subframe misalignment between different TAGs.        
An LTE Release-10 UE performs random access only on the PCell (i.e., all configured serving cells derive their UL timing from the PCell timing). For LTE Release-11, it has been agreed that when a SCell-TAG is configured:                there is one timing alignment timer (TAT) per TAG;        non-contention based RA is adopted for a SCell;        when the TAT associated with the PCell expires, all TAT's are considered expired (i.e., the UE flushes all Hybrid Automatic Repeat Request (HARQ) buffers, clears any configured assignments/grants, and Radio Resource Control (RRC) releases PUCCH/SRS for all configured serving cells; and        when the TAT associated with an SCell TAG expires:                    SRS transmissions in the SCell TAG shall be stopped and the Type 0 SRS configuration is released;            Channel Quality Indicator (CQI)/Precoder Matrix Indication (PMI)/Rank Indicator (RI) reporting configuration for the SCells is maintained; and            Medium Access Control (MAC) flushes all uplink HARQ buffers for the concerned SCells.                        
For LTE Release-10 CA, a UE that is not configured for simultaneous PUCCH/PUSCH transmission transmits uplink control information (UCI) on the PUCCH if it is not scheduled for PUSCH transmission. Conversely, the UE multiplexes Uplink Shared Channel (UL-SCH) data and UCI on the PUSCH if it has an UL grant. On the other hand, if an LTE Release-10 UE is configured for simultaneous PUCCH/PUSCH transmission it can, for example, transmit HARQ-acknowledgement (ACK) feedback on the PUCCH and multiplex CSI with UL-SCH data on the PUSCH in the same subframe. In case the total required power for a subframe would exceed the maximum power capability, an LTE Release-10 UE shall prioritize UL transmission as follows:
PUCCH>PUSCH multiplexed with UCI>PUSCH without UCI.
In other words, PUCCH has the highest priority, followed by PUSCH multiplexed with UCI, and then PUSCH without UCI has the lowest priority.
Furthermore, PUSCH power is scaled down while the PUCCH power is preserved in order to meet the UE's maximum transmit power capability PCMAX (i) in subframe i.
Adopting the LTE Release-10 non-contention based random access, the UE may be scheduled for PRACH transmission on a SCell via a Physical Downlink Control Channel (PDCCH) order. It is assumed that there is no PUSCH/SRS transmission in any SCell of the SCell-TAG because the UE is not UL-synchronized for the SCell-TAG. Therefore, the main issue for further study are the permitted combinations of PRACH on a SCell and PUCCH/PUSCH/SRS in a different TAG(s).